Let Over Lambda—50 Years of Lisp

by Doug Hoyte

Anaphoric Macros

More Phors?

Some of the most interesting macros from Paul Graham's On Lisp are anaphoric macros. An anaphoric macro is one that deliberately captures a variable from forms supplied to the macro. Thanks to their transparent specifications, these deliberately captured variables allow us windows of control over the macro expansion. Through these windows we can manipulate the expansion through combinations.

The classic anaphora like those in On Lisp are named after the literal words1A U-language quotation.anaphor and its plural, anaphora. An anaphor is a means of capturing a free U-language word for use in subsequent U-language. In programming terms, implementing a classic anaphor means finding places in your code—or in code you would like to write—where expressions can benefit from being able to refer to the results of previous, related expressions. Graham's anaphora and associated code are worth careful study. Especially see the defanaph macro[ON-LISP-P223] which enables some interesting types of automatic anaphor programming.

After some period of use, alambda has been found to be the most useful of the anaphoric macros in On Lisp. It is also one of the most simple and elegant demonstrations of an anaphoric macro and its intentional variable capture.

With alambda we capture the name self so that we can use it to refer to the very anonymous function we are building. In other words, recursing is as simple as a call to self. For example, the following function returns a list2If the condition is false, an absent tertiary clause on an if form returns nil, which is a list. of the numbers from n down to 1:

(alambda (n)
(if (> n 0)
(cons
n
(self (- n 1)))))

Alambda lets our code be intuitive and easy to read, and allows us to change our mind about whether an anonymous function should be able to call itself as easily as adding a single letter3This is another reason to not sharp-quote lambda forms. Changing a sharp quoted lambda form to an alambda form also requires deleting two characters.. Because of alambda's transparent specification for the self binding—and the fact that the only reason to ever use alambda is to make use of this binding—unwanted variable capture is never a problem.

Another handy anaphoric macro from On Lisp is aif, a macro that binds the result of the test clause to it for the true (secondary or consequent) clause to make use of4Exercise: Why would the false (tertiary or alternant) clause never make use of this anaphor?. Aif makes use of a valuable COMMON LISP feature: generalised booleans. In COMMON LISP, any non-nil value is a true boolean value so COMMON LISP programmers typically embed interesting information in truth values. Languages that have reserved true and false values—notably Scheme—employ explicit booleans, which sometimes force you to throw out extra information to satisfy redundant type constraints. Scheme has actually added a kludge to allow if, cond, and, or, and do to accept non-boolean5According to the Scheme boolean? predicate. values[R5RS-P25]. COMMON LISP, of course, is designed right—everything is a boolean.

It must also be pointed out that aif and alambda, like all anaphoric macros, violate lexical transparency. A fashionable way of saying this is currently to say that they are unhygienic macros. That is, like a good number of macros in this book, they invisibly introduce lexical bindings and thus cannot be created with macro systems that strictly enforce hygiene. Even the vast majority of Scheme systems, the platform that has experimented the most with hygiene, provide unhygienic defmacro-style macros—presumably because not even Scheme implementors take hygiene very seriously. Like training wheels on a bicycle, hygiene systems are for the most part toys that should be discarded after even a modest level of skill has been acquired.

Yes, there are many interesting things we can do with deliberate variable capture. There are a lot more phors. This book and Graham's On Lisp describe a only a tiny fraction of the potential inherent to this technique. Many more incredible inventions will come out of the intelligent application of anaphoric macros.

After a brief interlude into anaphora introduced by read macros, the remainder of this chapter describes a modest, specific application of anaphora related to one of the central themes of this book: the lexical closure—let over lambda. Most of this chapter describes interesting anaphoric macros for customising, adapting, and extending closures. Although the topics are very practical for use in real code, their main purpose is as a platform for discussing the properties and variations of anaphoric macros. Using macros to extend the concept of a closure is currently a hot research topic[FIRST-CLASS-EXTENTS][OPENING-CLOSURES].

Sharp-Backquote

Although most anaphora are introduced by regular macros, read macros also have the potential to introduce code that invisibly creates bindings for us. When read macros do so, they are called read anaphora. This section presents one such read macro that, while itself very modest, surprised even myself by turning out to be one of the most consistently useful throughout this book. I have tried to introduce this macro as soon as possible so that it can be used for the remainder of the code. Already, several macros shown should have used it.

Sharp-backquote is a read macro that reads in as a lambda form. By default, this lambda form will take exactly one argument: a1. The read macro then recursively invokes the read function with the supplied stream. Here is an example with the evaluation stopped (by quote) so we can observe the transparent introduction of the read anaphor6The prefix of the captured symbol, "a", of course stands for anaphor.:

* '#`((,a1))
(LAMBDA (A1)
`((,A1)))

This read macro abstracts out a common macro pattern. For example, if we have a list of variables and would like to make a list of let bindings that bind each variable to a symbol, say, empty, we can use mapcar like so:

The reason for the 1 character in the symbol a1 above is that users of the read macro can introduce a variable number of anaphora depending on the number provided to the numarg parameter of the read macro:

* '#2`(,a1 ,a2)
(LAMBDA (A1 A2)
`(,A1 ,A2))

So we can mapcar sharp-backquote expressions across more than one list at a time:

Another way to think about sharp-backquote is that it is to list interpolation as the format function is to string interpolation. Just as format lets us use a template with slots that are to be filled with the values of separate arguments, sharp-backquote lets us separate the structure of the list interpolation from the values we want to splice in. Because of the earlier described duality of syntax between lambda forms in the function position of a list and the lambda forms that use the lambda macro to expand into a function, we can also use sharp-backquote as the first element in a function call:

Unlike format, sharp-backquote doesn't use sequential positioning. Instead it uses the number on our anaphoric bindings. As a consequence, the order can be mixed up and we can even splice in bindings more than once:

Exercise: The references to the gensym #:G1735 look like they point to the same symbol but, of course, you never really can tell with gensyms by looking at their print names. Are these symbols eq? Why or why not?

Alet and Finite State Machines

With lambda and if there is only one useful anaphoric configuration. But the most interesting types of anaphoric macros make use of expansions in unforeseen ways. This section—even most of this chapter—is based around one such macro: alet. What extra bindings could be useful to forms inside the body of a let form? The very purpose of let is to create such bindings so capturing the variable introductions given to a let form is already done. However, a macro enhancement to let can have complete access to all the forms given to it, even the body of expressions intended to be evaluated with the new bindings. So what is the most useful part of the body? In most cases it is the last form in the body since the results from that form will be returned from the let statement itself7Because let provides an implicit progn.. We've seen that when we return a lambda expression that references these bindings created by let, the result is a lexical closure—an object frequently stored and used to later access the variables in the let statement. So, extending our closure-object analogy, the alet% macro acts exactly like the let special form except that it captures the symbol this from the body and binds it to the last expression in the form's body—the one that will be returned as the closure8Setq is used so that the form bound to this is defined in the lexical scope of the other arguments given by letargs..

Alet% can be useful when we have initialisation code in the lambda form that we don't want to duplicate. Because this is bound to the lambda form that we are returning, we can execute it before we return it from the enclosing let. The following is a closure whose construction shows a simple example use of alet% that avoids duplicating its reset and initialisation code:

Note that alet% changes the evaluation order of the forms in the let body. If you look at the expansion you will notice that the last form in the body is actually evaluated first, and its results are then bound to the lexical binding this before the preceding forms are evaluated. As long as the last argument is a constant, however, this re-ordering makes no difference. Remember that a lambda expression9Dlambda expands into lambda forms. is a constant value and is thus perfectly suitable for use in alet%.

As with many macro enhancements, because of the many degrees of freedom available, improvements to this macro are counterintuitive. Although there are many possibilities, this section considers one such specific improvement. Alet% can be made to not return the last form in its body—which we anticipate to be a lambda form—but instead a function that looks up another function inside the let form's lexical scope, then calls that function instead. This is sometimes called indirection because instead of returning a function to do something, we return a function that looks up a function using a pointer dereference, then uses that function instead. Indirection is a concept ubiquitous throughout programming languages for good reason. It lets us change things at run-time that, without indirection, are fixed at compile-time. Lisp lets us use indirection in a more succinct and efficient manner than many other programming languages. Alet, a version of alet% with indirection added, allows the function we returned as the closure to now be accessed or replaced by code inside the alet body, or, if we use dlambda, as will be shown soon, even outside the alet body.

Now that we can change the function that gets executed when invoking a closure with our alet macro we can create a pair of mutually referential functions using a pattern called alet over alambda. As long as all the states go back to the original state—instead of going to each other—alet over alambda is a convenient way to specify nameless state machines.

The following is a typical counter closure that takes an argument n and can have its direction toggled between increment and decrement by n when we pass the symbol invert as the argument instead of a number:

But we can change the actual function to be called to the internal lambda expression in our definition by passing the symbol invert to the closure:

* (alet-test 'invert)
#<Interpreted Function>

And now we're going down:

* (alet-test 3)
7

Finally, thanks to the self binding provided by alambda, we can again change the function to be called by passing the symbol invert:

* (alet-test 'invert)
#<Interpreted Function>

Back where we started, going up:

* (alet-test 5)
12

This closure has been bound in the function namespace to the symbol alet-test. But this closure is slightly different than a regular closure. While both this closure and regular closures are pointers to a single environment, one that can have any number of references to it, this closure uses indirection to change which piece of code gets run when it is invoked. Although any piece of code can be installed, only ones in the lexical scope of the alet, the one with the this anaphor available, can access its lexical bindings. But there is still nothing to prevent us from installing a new closure, with its own lexical bindings and perhaps with changed behaviour in the indirection environment installed by alet. Much of the remainder of this chapter is about useful things we can do with indirection environments created by alet.

A common macro technique is informally known as turning a macro inside out. When you turn a macro inside out you pick a typical form that uses a macro similar to the macro you would like to create, and expand it. You then use that expansion as a template for your desired macro. For example, we would like a more general method of creating closures with multiple states than the alet over alambda counter presented earlier. Here is the above inside out expansion of the invertible counter alambda use case:

From this, we notice that alambda can use the labels special form to make all of its bindings available to all of the bodies of its functions. What's more, we now have a fairly complete template for our eventual macro.

Alet-fsm gives us a convenient syntax for expressing multiple possible states for our closure to exist in. It is a very light sugar coating of macros over top of labels, combined with a code-walkingmacrolet transformation that allows us to pretend as though we have a function, state, that can change the closure's current state, accessed through the this anaphor provided by alet. As an example, here is a cleaner version of our invertible counter:

Alet-fsm is an instance of a technique we haven't seen before: anaphor injection. The use of this anaphor violates lexical transparency in so many ways that it is actually, somehow, lexically invisible. Not only does alet bind this for us invisibly, but the use of this by our alet-fsm macro is similarly invisible. Alet-fsm injects a free variable into our lexical context without us being able to see it at all in the lexical context.

The stylistic issues of this are uncertain11As are, by nature, all stylistic issues. Once something is perfectly understood, style becomes irrelevant. Free variable injection is not yet perfectly understood., but macro programming is, of course, not about style. It is about power. Sometimes free variable injection can create a symbiosis between two macros—one that can better programmatically construct expansions than can two isolated expansions. Because this type of macro programming is sophisticated, parallels can again be drawn to the C pointer analogy. Just as learning C pointers breeds dubious stylistic advice, so does free variable injection.

The most plausible hypothesis for the source of difficulty in understanding free variable injection is its fail-safe behaviour12Safe in the sense that, contrary to the real world, failing as quickly and as loudly as possible is safest.. With an anaphor, if the supplied user code doesn't make use of the binding the code will probably continue to function, whether you intended it to or not. It has, possibly, failed silently and thus un-safely. However, when you inject a free variable and there is no environment there to capture it, your entire expression has become free. When this happens, you need to decide what to do before you can evaluate that expression. It has failed safe.

Style aside, free variable injection is sometimes just what we need when we want two related macros to communicate back and forth. Injection is really the same operation as that performed by anaphora, just in the opposite direction. Because you are opening up a new channel of communication between your macros, the complexity issues scale even more quickly. Consider sitting in a house full of fragile glass. You can safely throw objects to people outside the house, even if they don't bother catching them, but you had better make sure you catch any objects thrown back at you.

Indirection Chains

There are many ways we can take advantage of the this anaphor provided by alet. Since the environment is accessed through a dummy closure that forwards all invocations to the real closure pointed to by this, we can pass the dummy closure reference around, copying it as often as needed. Indirection like this is useful because we can change what happens when this dummy closure is invoked without having to change references to the dummy closure.

Ichain-before is intended to be expanded in an alet form. It adds a new body of code to be executed before invoking the main closure. Going back to our counter example, ichain-before lets us add a new closure that prints out the previous value of the closed-over acc variable before it goes ahead and increments it:

Each addition of a new link in the chain adds the link to the very beginning of the chain, resulting in the links being visited in the reverse order from which they were added:

* (funcall * 2)
C
B
A
2

Statically adding indirection chains is sometimes useful when changing macros to avoid re-structuring macros by adding new surrounding code. But the most interesting possibilities for indirection chains pop up when we add them dynamically. Because we can create new closures at run-time and because we can access the internals of a closure through an anaphor, we can re-write how functions work at run-time. Here is a simple example in which every invocation of the closure adds another bit of code that prints "Hello world" when run:

* (loop for i from 1 to 4
do
(format t "~:r invocation:~%" i)
(funcall * i))
first invocation:
second invocation:
Hello world
third invocation:
Hello world
Hello world
fourth invocation:
Hello world
Hello world
Hello world

The ichain-after macro is similar to the ichain-before macro except it adds the closures to the other end of the execution chain: after the main closure has been invoked. Ichain-after uses prog1, which executes its provided forms consecutively and then returns the result of evaluating the first form.

Ichain-before and ichain-after are macros that inject free variables into their expansion. They inject the symbol this which we rely on being captured by the expansion of an alet macro. This sort of injection of symbols might seem to be bad style or error-prone, but it is actually a common macro technique. In fact, almost all macros inject symbols into the expansion. For instance, along with this, the macro ichain-before also injects symbols like let, setq, and lambda to be spliced into wherever the macro is expanded. The difference between symbols like this and pre-defined symbols like setq is that while lambda always refers to a single well-understood ANSI macro, symbols like this can refer to different things depending on the environments in which they are expanded.

Ichain-before and ichain-after are useful for tagging on code for a closure to run before or after the execution of the original closed-over expression but are by no means the only things we can do with the this anaphor. Another common task is checking for the validity of closure data after an invocation of the closure.

Ichain-intercept% is another macro designed to be used inside an alet form. The idea is that we would like to be able to intercept invocations of the closure and verify that the actions they performed didn't cause some sort of inconsistent state in the closure.

When the counter falls below 0, the code installed by ichain-intercept% will warn us:

* (funcall * -8)
Acc went negative
0

The counter was reset back to 0:

* (funcall ** 3)
3

The most interesting thing about ichain-intercept% is that it introduces a block anaphor named intercept. To use this anaphor we use return-from. The block will return this value from the closure invocation, intercepting the original value.

Instead of capturing the block anaphor intercept, ichain-intercept creates a local macro that allows the code inside ichain-intercept to use intercept to expand into a return-from where the block is specified by a gensym.

Of course, introducing all these closures transparently into operations can affect run-time performance. Luckily, modern lisp compilers are very good at optimising closures. If your application can stand a few pointer dereferences—and often it can—indirection chains might just be the best way to structure it. See section 7.4, Pointer Scope for another interesting way to think about indirection chains. Also see CLOS's before, after, and around functionalities.

Hotpatching Closures

There are three purposes for this important section. First, another interesting use of the this anaphor from alet is described. Second, the pattern alet over dlambda is discussed. Finally, a useful macro technique called anaphor closing is introduced.

In order to clearly illustrate anaphor closing, we will not work with the alet macro but instead an inside out expansion. Alet-hotpatch% is an expansion of alet with a special lambda form provided. This lambda form checks the first argument13With a pointer comparison. to see if it is the keyword symbol :hotpatch and, if so, replaces the indirected closure with another provided argument.

Being able to change the closure used in another forwarding closure at run-time is known as hotpatching. For instance, here we create a hotpatchable closure and store it in the symbol-function cell of the symbol hotpatch-test for later use:

Haven't we seen this sort of run-time destructuring on keyword symbols before? Yes, in fact we wrote a macro for doing exactly this in section 5.7, Dlambda. Alet-hotpatch is a version of alet-hotpatch% that takes advantage of dlambda. Sometimes without even realising it, by using macros we wrote previously in the definition of new macros, we are performing macro combination. With well designed macros the expansion can be fully understood and, although it might violate lexical transparency in many ways, no combination problems emerge because all components fit together meaningfully.

Alet-hotpatch creates a hotpatchable closure but there is one slight conceptual flaw. Because the only real reason for using alet-hotpatch is to create this sort of hotpatchable closure, we might forget that it also introduces the anaphor this into the scope of the forms provided. When we forget about anaphora we've created, we risk unwanted variable capture problems. To avoid these problems, we might choose to employ a technique known as anaphor closing. When we close an anaphor, we don't need to change the way our anaphoric macros function, just restrict them in the ways they can be combined.

Because we have turned the alet expansion inside out, we can lexically see the creation of the this anaphor in the definition of alet-hotpatch. And because alet-hotpatch also contains the code using the this anaphor to implement hotpatching, we can close the anaphor so that the symbol this is no longer captured by the macro. How do we normally avoid introducing unwanted bindings? We name the bindings using gensyms of course.

Let-hotpatch is an example of closing the this anaphor into a more contained version—a safer version for when hotpatching is all that is required. The leading a was removed from the name to suggest that this new macro no longer introduces an anaphor into the supplied body of code. Of course if we wanted to refer to this for some reason other than hotpatching, we should have left the anaphor open.

This technique of opening and closing anaphora becomes second nature after you have written enough of such macros. Just like we can write macros that inject free variables into an expansion without thinking about how we will capture them until we write the lexical context in which they will be expanded, we sometimes choose to leave an anaphor open while developing macros to experiment with combinations of anaphoric macros and free variable injection macros. Once the most useful combinations are found, we can merge the macros together and replace all anaphora used during development with with gensyms. Like let-hotpatch does, this technique can use defmacro! to move the anaphor's scope from the macro expansion to the macro definition. Instead of lexically introducing an anaphor, we introduced another type of anaphor—one that doesn't take effect in the full lexical scope of the expansion but only in another, more limited scope. This scope is described further in the following section.

Sub-Lexical Scope

Our defmacro! macro-defining macros that we defined in section 3.5, Unwanted Capture look for the presence of automatic gensyms in the provided code using Graham's flatten utility. Now is the time to confess a small lie we have been telling through this book. Before now, because we hadn't explained free variable injection and anaphora, we pretended that the G-bang symbol names in defmacro! definitions are applicable in the lexical scope of the macro definition. This is actually not true—defmacro! provides these bindings under a slightly different type of scope called sub-lexical scope.

Remember that scope means where references to a variable are valid and lexical scope means that the name is applicable to code in the textual body of a binding construct such as let. The important distinction between lexical scope and sub-lexical scope is that for lexical scope this includes all macroexpansions of code in the let body. So describing lexical scope as creating variables only accessible to code in the textual body of a binding construct is actually a lie too—macros can inject variable references. Such variables are injected from outside the textual body of the binding construct.

Implementing genuine textual scoping by limiting the possible ways to access lexical variables results in sub-lexical scope. References to a sub-lexically scoped variable are only valid if the symbols representing them occur in the raw lists that were passed to lisp before macro-expansion.

Because defmacro! pre-processes the code it was given and creates the list of all the G-bang symbols before the code gets expanded, the G-bang symbols are sub-lexically bound. We can't write macros that inject G-bang symbols into defmacro! because lexical bindings for the G-bang symbols were never created. Here is a typical use of a sub-lexical G-bang symbol:

* (defmacro! junk ()
`(let ((,g!var))
,g!var))
JUNK

Both G-bang symbols were found in the sub-lexical scope of defmacro! so expansion is as we would expect:

* (macroexpand '(junk))
(LET ()
(LET ((#:VAR1663))
#:VAR1663))
T

However, in order to explore the concept of sub-lexical scope, we will define a macro that injects a G-bang symbol:

* (defmacro injector-for-g!var ()
''g!var)
INJECTOR-FOR-G!VAR

Now we can write junk2. Junk2 is identical to junk except that we have replaced our G-bang symbols with a macro that expands into a G-bang symbol:

Sub-lexical scoping turns up surprisingly often in complex macros. As well as defmacro!, we've seen it in at least one other example: the with-all-cxrs macro from section 5.6, Recursive Solutions sub-lexically binds list accessor functions. The consequence of sub-lexical binding is that we can't refer to such bindings from macro expansions. Sometimes this access limitation is useful, sometimes not. In with-all-cxrs, sub-lexicality could be considered undesirable. When our accessor is in with-all-cxrs's sub-lexical scope, there is no problem:

* (with-all-cxrs
(cadadadr nil))
NIL

And we can even write macros that expand into these accessors, as long as the macro definitions are in the sub-lexical scope of with-all-cxrs:

Now that we are familiar with anaphora and have seen numerous examples of complex macros—including some that utilise sub-lexical scope—we can discuss an interesting theoretical macro: sublet. This macro is designed to create sub-lexical bindings for code using a syntax similar to the usual let form syntax. The discussion of sublet, as with many lisp macros, begins with a utility.

Let-binding-transform is a simple utility that handles the case of a let form binding being a single symbol. In the following, a is normalised to (a):

* (let-binding-transform
'(a (b) (c nil)))
((A) (B) (C NIL))

Sublet also uses the tree-leaves utility we defined in section 5.3, Implicit Contexts. Recall that the tree-leaves macro takes three arguments: an arbitrary list structure, an expression that can make use of an x variable to determine whether a leaf should be changed, and another expression that can make use of a different x to determine what valid leaves should be changed to.

The choice to implicitise the bindings of x with the same name turns out to be a useful duality of syntax. When we can't factor common code in an expression the usual way, sometimes we can gain this brevity advantage by using syntactic duals in other ways. The definition of sublet uses the self-referential read macros described in section 4.5, Cyclic Expressions. Especially for things like accessors that can change many times throughout the writing of a program, read macros allow us to have one and only one form representing the accessor. Thanks to our use of implicitisation with the tree-leaves macro, it is easy to find and understand the code duplication because the code is close together.

Sublet takes the form representing the let bindings and applies our let-binding-transform utility, generating new list structure in the process. It then prepends14Prepends instead of appends so we can still support bindings without default values, such as (a). a gensym to each binding with a print name corresponding to the binding name. Sublet expands into a let form which binds these gensym symbols to the values passed into the binding form, then uses tree-leaves to replace all occurrences of the binding name symbols in the provided code with their corresponding gensyms. Sublet does not expand any macros or parse any special forms in the body to look for occurrences of these binding name symbols because sublet creates sub-lexical bindings. For example, if all references to a are sub-lexical, it will replace them with gensyms:

However, because sub-lexical scope doesn't involve expanding macros, and thus necessarily doesn't involve interpreting special forms like quote, instances of the symbol a that aren't supposed to be variable references are also changed:

Sub-lexical scope takes effect before list structure is interpreted as lisp code by your system's code-walker. That is an important observation—one with ramifications still not completely explored. Sublet interprets your code differently than does the code-walker provided with COMMON LISP.

Here we are standing on one of the many edges of macro understanding. What sort of interesting types of scoping lie between unexpanded sub-lexical scope and fully expanded lexical scope? For lack of a better name, we will call this infinitely large category of scopes super sub-lexical scopes15I am giving it this silly name because I expect better names to become obvious when the concept is better understood..

A fairly obvious super sub-lexical scope uses sublet*. This macro uses sublet underneath but changes each form in the body by macro expanding them with the macroexpand-1 function. Now, instead of appearing in the raw list structure, references to symbols must occur after the first step of macro expansion. This type of super sub-lexical scope allows a macro in each of the let form's body to inject or remove references from the scope. If the macros don't do either of these things—or if the forms aren't macros at all—this type of super sub-lexical scope acts just like sub-lexical scope:

So a is not captured sub-lexically16Walker:macroexpand-all is a CMUCL component of a complete code-walker.:

* (walker:macroexpand-all *)
(LET ((#:A1666 0))
(LIST A))

With sublet and sublet* we can control at what level of macro expansion the a variable is considered valid by using sub-lexical or super sub-lexical scopes. As mentioned above, super sub-lexical scope is actually an infinite class of scopes, one that is almost completely unexplored intellectually. As many ways as there are to walk code (a lot) there are super sub-lexical scopes. This class of scoping leads into another category of mostly unexplored macros: macros that change how lisp macros work, when they are expanded, where references are valid, how special forms are interpreted, etc. Eventually, a macro-programmable macro expander.

Pandoric Macros

Pandora's box is a Greek myth about the world's first woman: Pandora. Pandora, the U-language symbol, translates from Greek into all-gifted. Pandora, the woman, was tempted by curiosity to open a small box which irreparably unleashed all of humanity's evil and sins upon the world. While the macros described in this section are very powerful and might teach you a way of programming you never forget, rest assured that our outcome will be far better than poor Pandora's. Open the box.

First we take a slight detour through another famous lisp book: Lisp in Small Pieces[SMALL-PIECES] by Christian Queinnec. Queinnec is a widely respected lisp expert and has contributed much to our lisp knowledge. Queinnec's book is about implementing compilers and interpreters of varying sophistication in and for the Scheme programming language17Though it sometimes describes other lisps and their features..

In Lisp in Small Pieces there is a short but interesting discussion on macros. Much of it relates to describing the different macro system variations possible thanks to the ambiguity of the Scheme macro specification18Thanks, but no thanks., but there are also a few interesting notes on why we might want to use macros and how to go about using them. If you have read and understood chapter 3, Macro Basics, most of the macros presented in the chapter of Lisp in Small Pieces about macros will, to you, belong in the trivial category, save one enticing macro that we will now discuss.

Like many programming books, Lisp in Small Pieces takes us to and leaves us at an implementation of a system for object-oriented programming. Usually these implementations serve to outline a subset of CLOS, the COMMON LISP Object System. Queinnec calls his subset MEROONET. Queinnec remarks that when defining a method for a MEROONET class it would be nice to be able to directly refer to the fields of the object being defined instead of using accessors. In Queinnec's (translated) words[SMALL-PIECES-P340-341]:

Let's take the case, for example, of the macro with-slots from CLOS; we'll adapt it to a MEROONET context. The fields of an object—let's say the fields of an instance of Point—are handled by read and write functions like Point-x or set-Point-y!. It would be simpler to handle them directly by the name of their fields, x or y, for example, in the context of defining a method.

Here is Queinnec's desired interface (which he has called define-handy-method) defining a new method, double:

In other words, it would be nice if we could use macros to access foreign bindings—in this case object slots—as if they were lexical bindings. Although this is undeniably useful for abbreviation purposes, its most important implication is its ability to give dualities of syntax to our existing and future macros.

As Queinnec notes, COMMON LISP implements this functionality for CLOS with a macro called with-slots. This is an example of COMMON LISP doing what it was designed to do: allowing abstractions based on a refined, standardised macro system. While most languages are designed to be easy to implement, COMMON LISP is designed to be powerful to program. Queinnec's conclusion was that language limitations make this mostly impossible in Scheme, especially where portability is required:

[L]acking reflective information about the language and its implementations, we cannot write a portable code-walker in Scheme, so we have to give up writing define-handy-method.

Although COMMON LISP still allows a large number of legal ways to implement macro systems, it is designed to provide general meta-programming tools that come together in standard and portable ways. The two advanced COMMON LISP macro features that allow us to implement things like CLOS's with-slots are generalised variables and symbol macros. This section takes advantage of an opportunity to show off this wonderful confluence of COMMON LISP features as well as to bring together everything we have seen so far regarding anaphoric macros, in the process discovering an interesting class of macros called pandoric macros.

The idea behind pandoriclet is to open closures, allowing their otherwise closed-over lexical variables to be accessed externally. As with some of our previous macros like alet-hotpatch, pandoriclet compiles an indirection environment that chooses different run-time behaviours depending on the arguments passed.

We again started with an inside out expansion of alet, keeping in mind the introduction of an anaphor called this. Pandoriclet is similar to other macros we've seen. As with all of our anaphoric let variants, we assume the final form in the pandoriclet body will be a lambda form. Like alet-hotpatch, pandoriclet uses the dlambda macro to dispatch between different possible pieces of code to execute when the closure returned from pandoriclet is invoked. Pandoriclet uses the let-binding-transform utility function introduced in the previous section to deal with null bindings created—like (let (a) ...). This utility function is necessary to pandoriclet for the same reason that it was necessary for sublet: these macros code-walk the bindings provided to let where our previous macros blindly spliced the bindings into another let.

We have put in two calls to list-creating utility functions yet to be defined: pandoriclet-get and pandoriclet-set, each of which accept a list of let bindings. Notice we can reference functions that don't yet exist as long as we define them before the macro is expanded which obviously can't occur before we use the macro. Using auxiliary functions to help with defining macros is a good habit to pick up. Not only can it make your definitions more readable, but also can help when testing components of the macro and can prove useful in future macros. The best part about this sort of abstraction is that, as when combining macros, we keep our lexical context available for utilities to make use of.

So, remembering the lexical context, we write pandoriclet-get and pandoriclet-set. For pandoriclet-get, we remember that dlambda has bound a variable sym around where our list will be spliced in. We use sym in a case form that compares it to the symbols that were passed to pandoriclet19Recall that case with symbols compiles to a single pointer comparison per case.. If we find a symbol, the current value of the binding it refers to is returned. If not, an error is thrown. Pandoriclet-set is nearly identical, except that dlambda bound one extra symbol for it to use: val. Pandoriclet-set uses setq to change the binding referred to by sym to val.

Pandoriclet provides the same interface as all our anaphoric let variants so we can use it to make our usual counter closure:

However, now we have direct access to the binding that was called acc when the closure was created:

* (pantest :pandoric-get 'acc)
8

And we can similarly change the value of this binding:

* (pantest :pandoric-set 'acc 100)
100
* (pantest 3)
103

Even the value of our this anaphor is accessible, since we deliberately left the anaphor open and added the symbol this to the letargs binding list when the macro was expanded:

* (pantest :pandoric-get 'this)
#<Interpreted Function>

So this closure we've created with pandoriclet is actually no longer closed. The environment used by this closure—even when all lexical variable symbols have been removed by the compiler—is still accessible through our anonymous function returned from pandoriclet. How does this work? With pandoric macros, additional code is compiled in to provide a way to access the closure from outside. But the power of pandoric macros can't be seen by looking at this low-level view of what is happening. What we have done is created an inter-closure protocol, or message passing system, for communicating between closures.

Before we continue with pandoric macros, first we need to point out one of the most important examples of duality of syntax in COMMON LISP: generalised variables. The details of this are complicated, and I won't describe all of them here. For that I recommend Graham's On Lisp which has the best treatment I am aware of. The finer points are subtle the idea is simple: accessing a generalised variable is syntactically dual to setting it. You have only one setter form, setf, which is capable of setting all types of variables by using the same syntax you would use to access them.

For example, with a regular variable you usually access its value through its symbol, say, x. To set it, you can use (setf x 5). Similarly, to access the car slot of a cons called, say, x, you use (car x). To set it, you can use the form (setf (car x) 5). This hides the fact that the actual way to set a cons is to use the function rplaca. By implementing this duality of syntax we cut in half the number of accessors/setters we need to memorise and, most importantly, enable new ways for us to use macros.

The function get-pandoric is a wrapper around the inter-closure protocol getter syntax. It is declaimed inline to eliminate any performance impact caused by this wrapping.

Defsetf is an interesting COMMON LISP macro not entirely unlike our defmacro! extension to defmacro in that it implicitly binds gensyms around provided forms. Defsetf works great for defining the setter side of a generalised variable duality as long as the getter can be expressed as a function or macro that evaluates all of its arguments exactly once. Note that while get-pandoric could have been defined as a macro, the only reason to do that would be for inlining purposes. Macros are not for inlining, compilers are for inlining.

So going back to our pandoric counter stored in the symbol-function of pantest, we can use this new getter function to retrieve the current value of pantest's acc binding:

* (get-pandoric #'pantest 'acc)
103

And now, thanks to generalised variables and defsetf, we can use a syntactic dual to set it:

* (setf (get-pandoric #'pantest 'acc) -10)
-10
* (pantest 3)
-7

The environments that close over functions—what we've been calling the let in let over lambda—are starting to look like regularly accessible generalised variables, just like a cons cell or a hash-table entry. Closures are now even more first-class data structures than they used to be. Bindings that were previously closed to outside code are now wide open for us to tinker with, even if those bindings were compiled to something efficient and have long since had their accessor symbols forgotten.

But any discussion of generalised variables is incomplete without a mention of its close relative: the symbol macro. Symbol-macrolet, like its name implies, allows us to expand symbols into general lisp forms. Since it is intuitive and more flexible to use forms that look like function calls to represent macro transformations20Symbol macros take no arguments so a symbol macro definition always expands the same., there isn't much use for symbol-macrolet save one important application for which it is vital: symbol macros let us hide generalised variables such that users of our macro think they are accessing regular lexical variables.

The introduction of symbol macros has resulted in one of the strangest kludges to the COMMON LISP language: normally when setting a variable accessed through a regular symbol, like (setf x t), setf will expand into a setq form because this is what setq was originally designed to do: set lexical and dynamic variables (which are always referred to by symbols). But the setq special form cannot set generalised variables, so when symbol macros were introduced and it became possible for symbols to represent not just a lexical/dynamic binding but instead any generalised variable, it was necessary to mandate that setq forms setting symbols with symbol macro definitions are to be converted back into setf forms. Strangely, this really is the right thing to do because it allows macros that completely hide the presence of generalised variables from the macro's users, even if they choose to use setq. The really right solution would be to remove the redundant setq from the language in favour of the more general setf, but this will not happen for obvious compatibility reasons and because during macro creation setq can also be a useful safety shortcut—setf plus a check that a symbol has been spliced in instead of a list form. When using setq for this remember that it only helps for its splicing safety property; as we've seen, a symbol can reference any generalised variable thanks to symbol-macrolet.

The with-pandoric macro expands into a symbol-macrolet which defines a symbol macro for each symbol provided in syms. Each symbol macro will expand references to its symbol in the lexical scope of the symbol-macrolet into generalised variable references using our get-pandoric accessor/setter to access the result of evaluating the second argument to the macro: o!box (stored in g!box).

So with-pandoric lets us peek into a closure's closed over variable bindings:

As per our design of using generalised variables to form a syntactic dual for the setting and getting of this variable, we can even pretend it is a regular lexical variable and set it with setq:

* (with-pandoric (acc) #'pantest
(setq acc 5))
5
* (pantest 1)
6

We have now looked at most of the pieces that make up pandoric macros. First, a macro for creating closures: pandoriclet, which captures an anaphor, this, referring to the actual function used when invoking the closure. This macro also compiles in some special code that intercepts certain invocations of this closure and instead accesses or modifies its closed-over lexical variables. Second, a single syntax for both accessing and setting an accessor is is implemented with get-pandoric and defsetf. Finally, the macro with-pandoric uses symbol-macrolet to install these generalised variables as seemingly new lexical variables with the same names as the closed-over variables. These variables refer to the original environment created with pandoriclet, but from separate lexical contexts.

As an example, we relate this ability to open up closures by comparing it to the hotpatching macros from section 6.5, Hotpatching Closures. Recall that alet-hotpatch and its closed anaphor cousin, let-hotpatch, create closures with an indirection environment so that the function that is called when the closure is invoked can be changed on the fly. The biggest limitation with these macros is that they force you to throw out all the lexical bindings that closed over the previous anonymous function when you hotpatched it. This was unavoidable because when we wrote those macros, closures were closed to us.

With alet-hotpatch and let-hotpatch, we had to compile special purpose code into each closure that was capable of setting the this anaphoric lexical binding to its new value. But since we can now open up a closure defined with pandoriclet and run this setter code externally, we can define a hotpatching function pandoric-hotpatch that will work with any pandoric closure.

Sometimes abstractions just feel right and it is hard to exactly say why. Probably because most programming is the disharmonious combining of disjoint parts, it is surprising and pleasant when you discover abstractions that—seemingly by chance—fit together perfectly. Pandoric-hotpatch just seems to read exactly like how it works: it opens a pandoric interface, takes the variable this from the lexical scope of the closure box, and then uses setq to set this to the closure to be hotpatched in, new.

We can even use pandoric-hotpatch on a pandoric closure we created before we knew we wanted it to be hotpatchable. Remember the counter closure we have been playing with throughout this section? It should still be bound to the symbol-function of the symbol pantest. We were at 6 and counting up when we left off:

* (pantest 0)
6

Let's install a new closure—one that has a new binding for acc starting at 100, and is counting down:

The macro pandoric-recode takes a slightly different approach to hotpatching. It conserves the original lexical environment of the code while still managing to change the function to be executed when the closure is invoked to something coded and compiled externally. Sound too good to be true? Remembering that the current value for acc is 6 in the original pandoric environment, we can use pandoric-recode to install a new function that makes use of this original value and, oh, let's say, decrements the counter by half the value of the provided n:

The macro we have used so far to create pandoric closures is pandoriclet. Plambda is an inside out re-write of pandoriclet that adds a few important features. First and foremost, plambda no longer creates the let environment to be used through our pandoric accessors. Instead, plambda takes a list of symbols that refer to variables that are expected to be in the caller's lexical environment. Plambda can export any variables in your lexical environment, transparently making them available for other lexical scopes to access—even ones written and compiled before or after the plambda form is.

This is an incremental improvement to our let over lambda closure system designed to maximise dual syntax. Thanks to pandoric macros, the most important of which are plambda and with-pandoric, we can easily and efficiently transcend the boundaries of lexical scope when we need to. Closures are no longer closed; we can open closures as easily as re-writing our lambda forms to be plambda forms. We use plambda to export lexical variables and with-pandoric to import them as completely equivalent lexical variables. In fact these new variables are so equivalent that they aren't even really new variables at all. A better way of thinking about pandoric variables are that they are simply an extension of the original lexical scope. As a simple example use of plambda, here is a pandoric counter that exports variables from two potentially different lexical environments:

Notice how easy it was to export these lexical references. Making a closure pandoric is as easy as adding a p character before the lambda and adding a list of variables to export after the lambda arguments. We can the open this closure—and any pandoric closure that exports the symbols a and b—by using with-pandoric like so:

Plambda is an example of how factoring out general components of macro expansions can be helpful. Remember when we wrote pandoriclet and decided to move the creation of case statements for the getter code to the function pandoriclet-get and the setter code to pandoriclet-get? Plambda makes use of these same functions. Even though these macros splice the results from these functions into fairly different lexical contexts, since both macros have been written to use the same variable naming convention and inter-closure protocol, the code is re-usable.

So pandoric macros break down lexical boundaries. They allow you to open up closures whenever needed and represent a beautiful confluence of a variety of COMMON LISP language features: anaphoric macros, generalised variables, and symbol macros. But what good are they, really?

Pandoric macros are important because they give us the main advantages of object systems like CLOS without requiring us to depart from the more natural let-lambda combination programming style. In particular, we can add functionality, or methods, for closures to use without having to re-instantiate instances of already created objects.

Make-stats-counter is a lambda over let over plambda we have created to create counters, except that it maintains three pieces of information. In addition to the sum, the sum of the squares, and the number of items so far processed are also kept. If we had used lambda instead of plambda in the definition of make-stats-counter, most of this information would be inaccessible to us. We would be locked out because these variables would be closed to us.

How do we write pandoric methods? We can simply access the variables using with-pandoric as we have demonstrated above, or, since this is lisp, design a more specific interface.

Defpan is a combination of the defun and with-pandoric macros. Defpan's main purpose is to enable a duality of syntax between function writing using defun and foreign lexical scope access using with-pandoric. Although we provide arguments to defpan using the same syntax as in lambda forms—a list of symbols—the arguments to defpan mean something different. Instead of creating a new lexical environment, these pandoric functions extend the lexical environment of the pandoric closures they are applied to. With defun and regular lambda forms, the name (symbol) you give a variable is unimportant. With pandoric functions, it is everything. Furthermore, with pandoric functions the order of the arguments doesn't matter and you can elect to use as few or as many of the exported lexical variables as you please.

Defpan also provides an anaphor called self that allows us to perform a useful technique called anaphor chaining. By invisibly passing the value of self between pandoric functions, we can maintain the value of this anaphor throughout a chain of function calls. As with all chaining constructs, be sure you don't end up in an infinite loop.

Three methods are presented that can be used on our closures created with make-stats-counter or any other pandoric closure that exports the necessary variable names. Stats-counter-mean simply returns the averaged value of all the values that have been passed to the closure. Stats-counter-variance computes the variance of these values by following a link in the chain and stats-counter-stddev follows yet another to compute the standard deviation. Notice that each link in the chain only needs to pass on the anaphor self to refer to the full lexical context of the closure. We see that the individual pandoric functions only need to reference the variables they actually use and that these variables can be referred to in any order we wish.

So plambda creates another anaphor—self. While the anaphor this refers to the actual closure that is to be invoked, self refers to the indirection environment that calls this closure. Although it sounds a bit peculiar, code inside our plambda can use self to pandorically access its own lexical environment instead of directly accessing it. This so far only seems useful for defpan methods that have been written to work inside our lexical scope.

Make-noisy-stats-counter is identical to make-stats-counter except that it uses the self anaphor to invoke our defpan functions stats-counter-mean, stats-counter-variance, and stats-counter-stddev.

Plambda and with-pandoric can re-write lexical scope in any way we please. We conclude this chapter with such an example. A limitation of lexical scope sometimes lamented upon is the fact that the COMMON LISP function eval will throw out your current lexical environment when it evaluates the form passed to it. In other words, eval evaluates the form in a null lexical environment. In COMMON LISP it couldn't be any other way: eval is a function. Here is the problem:

* (let ((x 1))
(eval
'(+ x 1)))
Error: The variable X is unbound.

Sometimes it would apparently be desirable to extend your lexical environment to eval. But be careful. Often it is said that if you are using eval you are probably doing something wrong. Misuse of eval can result in slower programs because eval can be a very expensive operation—mostly because it needs to expand macros present in the form passed to it. Should you suddenly find a need for eval when programming, ask yourself why you didn't do whatever it is you want to do a lot earlier. If the answer is that you couldn't have, say because you just read the form in, then congratulations, you have found one of the rare legitimate uses of eval. Any other answers will lead straight back to the way you probably should have done it in the first place: with a macro.

But let's say that you really do want to eval something, if only you could carry along that pesky lexical context. The pandoric-eval macro is a fun example use plambda and with-pandoric. Pandoric-eval uses a special variable that we have named pandoric-eval-tunnel to make a pandoric closure available to the eval function through the dynamic environment. We choose exactly which lexical variables to tunnel through the dynamic environment by providing a list of all their symbols as the first argument to pandoric-eval. Here it is applied to our earlier example:

* (let ((x 1))
(pandoric-eval (x)
'(+ 1 x)))
2

And the expression evaluated by pandoric-eval can modify the original lexical environment; pandoric-eval is a two way tunnel:

* (let ((x 1))
(pandoric-eval (x)
'(incf x))
x)
2

This section, although very lengthy, has still only scratched the surface of what is possible with pandoric macros and their many possible variations. I am looking forward to the many interesting future developments that will come out of them.

Exercise: Can pandoric-eval calls nest? That is, can you use pandoric-eval to evaluate a form that evaluates pandoric-eval? Why or why not?

Exercise: Although the implementation of pandoric macros here is fairly efficient, it could be improved. Try replacing pandoriclet-get and pandoriclet-set to generate code that uses a hash-table instead of case and benchmark these two implementations for small and large numbers of pandoric variables. Investigate your favourite CLOS implementation, imitate how dispatching is done there, re-benchmark.